The present invention relates broadly to flame retardant materials for use in electromagnetic interference (EMI) applications, and more particularly to an intumescent, pressure-sensitive adhesive composition particularly adapted for use in EMI shielding gaskets, tapes, wraps, and the like for attaching an electrically-conductive foil, fabric, or other sheath or jacket to a foam core, cable, or other substrate.
The operation of electronic devices including televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. Such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having a less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., xe2x80x9cGaskets That Block EMI,xe2x80x9d Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
Requirements for typical EMI shielding applications often dictate a low impedance, low profile gasket which is deflectable under normal closure force loads. Other requirements include low cost and a design which provides EMI shielding effectiveness both for the proper operation of the device and, at least in the United States, for compliance with commercial Federal Communication Commission (FCC) EMC standards and other governmental regulations.
A particularly economical gasket construction, which also requires very low closure forces, i.e. less than about 1 lb/inch (0.175 N/mm), is marketed by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass. under the tradename xe2x80x9cSoft-Shield(copyright) 4000 Series.xe2x80x9d Such construction consists of an electrically-conductive foil, fabric-reinforced foil, or fabric jacket or sheathing which is xe2x80x9ccigarettexe2x80x9d wrapped over a closed-cell polyurethane or other foam core profile which may be in the form of a generally elongate, strip-type gasket or, alternatively, in the form of a xe2x80x9cpicture framexe2x80x9d or other large surface area shape. As is described further in U.S. Pat. No. 4,871,477, polyurethane foams generally are produced by the reaction of polyisocyanate and a hydroxyl-functional polyol in the presence of a blowing agent. The blowing agent effects the expansion of the polymer structure into a multiplicity of open or closed cells.
The jacket typically is provided as a highly conductive, i.e., about 1 xcexa9-sq., nickel-plated nylon fabric or fabric-reinforced aluminum foil. The jacket may be machined wrapped over the core and bonded thereto via an interlayer of an acrylic or other pressure-sensitive adhesive (PSA). Similar gasket constructions are shown in commonly-assigned U.S. Pat. No. 5,028,739 and in U.S. Pat. Nos. 4,857,668; 5,054,635; 5,105,056; and 5,202,536.
Many electronic devices, including PC""s and communication equipment, must not only comply with certain FCC requirements, but also must meet be approved under certain Underwriter""s Laboratories (UL) standards for flame retardancy. In this regard, if each of the individual components within an electronic device is UL approved, then the device itself does not require separate approval. Ensuring UL approval for each component therefore reduces the cost of compliance for the manufacturer, and ultimately may result in cheaper goods for the consumer. For EMI shielding gaskets, however, such gaskets must be made flame retardant, i.e., achieving a rating of V-0 under UL Std. No. 94, xe2x80x9cTests for Flammability of Plastic Materials for Parts in Devices and Appliancesxe2x80x9d (1991), without compromising the electrical conductivity necessary for meeting EMI shielding requirements.
In this regard, and particularly with respect to EMI shielding gaskets of the above-described fabric over foam variety, it has long been recognized that foamed polymeric materials are flammable and, in certain circumstances, may present a fire hazard. Owing to their cellular structure, high organic content, and surface area, most foam materials are subject to relatively rapid decomposition upon exposure to fire or high temperatures.
One approach for imparting flame retardancy to fabric over foam gaskets has been to employ the sheathing as a flame resistant protective layer for the foam. Indeed, V-0 rating compliance purportedly has been achieved by sheathing the foam within an electrically-conductive Ni/Cu-plated fabric to which a thermoplastic sheet is hot nipped or otherwise fusion bonding to the underside thereof. Such fabrics, which may be further described in one or more of U.S. Pat. Nos. 4,489,126; 4,531,994; 4,608,104; and/or 4,621,013, have been marketed by Monsanto Co., St. Louis, under the tradename xe2x80x9cFlectron(copyright) Ni/Cu Polyester Taffeta V0.xe2x80x9d
Other fabric over foam gaskets, as is detailed in U.S. Pat. No. 4,857,668, incorporate a supplemental layer or coating applied to the interior surface of the sheath. Such coating may be a flame-retardant urethane formulation which also promotes the adhesion of the sheath to the foam. The coating additionally may function to reduce bleeding of the foam through the fabric which otherwise could compromise the electrical conductivity of the sheath.
In view of the foregoing, it will be appreciated that further improvements in the design of flame retardant, foam core EMI shielding gaskets, as well as sheathing materials therefore, would be well-received by the electronics industry. Especially desired would be a flame retardant gasket construction which achieves a UL94 rating of V-0.
The present invention is directed to an intumescent, flame-retardant pressure-sensitive adhesive (PSA) composition particularly adapted for use in foil or fabric-over-foam EMI shielding gasket constructions for bonding an electrically-conductive foam or fabric sheath or jacket to a polyurethane or other foam core. In being formulated to xe2x80x9cintumescexe2x80x9d when exposed to a flame environment, the PSA composition foams or otherwise exhibits a volumetric expansion in developing a thermally insulative, carbonific char.
When employed as an PSA interlayer to bond a fabric or foil wrap to a foam core in an EMI shielding gasket construction, such composition provides a protective, flame retardant layer for the gasket. Gaskets of such construction advantageously have been observed to achieve a UL94 rating of V-0 over a range of cross-section and even at narrow cross-sections down to about 1 mm in thickness. The inventive PSA composition also may be employed within other flame retardant, EMI shielding materials, such as tapes for the wrapping of cables and the like, as coated onto one side of an electrically conductive fabric, mesh, or foil layer.
In a preferred embodiment, the intumescent PSA composition of the present invention is formulated as an acrylic-based PSA which is rendered intumescent and flame retardant via its loading with a unique combination of flame retardant additives. Such combination includes a halogenated first flame retardant component, an metal oxide-based second flame retardant component, and a filler component of expandable, intercalated graphite particles. In a particularly preferred formulation, the halogenated first flame retardant component is a polybrominated diphenyl compound such as a decabromodiphenyl oxide or ether, the metal oxide-based second flame retardant component is antimony oxide, trioxide, or pentoxide, and the filler component is graphite flake.
Unexpectedly, it has been observed that such flame retardant additive combination functions synergistic. In this regard, each of the components separately has been observed not to impart flame retardancy to the PSA composition effective to achieve UL94 V-0 protection within a fabric or foil over foam EMI shielding gasket construction.
A feature of the present invention therefore is to provide a flame retardant, intumescent pressure sensitive adhesive (PSA) composition for EMI shielding gaskets, tapes, wraps, and the like. The composition is formulated as an admixture of: (a) a PSA component; (b) a halogenated first flame retardant component; (c) a metal oxide-based second flame retardant component; and (d) a filler component of expandable, intercalated graphite particles. In a preferred formulation, the PSA component (a) is acrylic-based, the halogenated first flame retardant component is a polybrominated diphenyl compound such as decabromodiphenyl oxide or decabromodiphenyl ether, the metal-based second flame retardant component (c) is antimony oxide, antimony trioxide, or antimony pentoxide; and the filler component (d) is graphite flake.
Another feature of the present invention is to provide a flame retardant, EMI shielding material for wrapping gaskets, cables, and the like. Such material is constructed as having an electrically-conductive layer with a first and second surface, and a flame retardant pressure sensitive adhesive (PSA) layer disposed on one or both of the first or second surface of the electrically-conductive layer. The PSA layer is formed of an intumescent PSA composition which, in turn, is formulated as an admixture of: (a) a PSA component which may be acrylic-based; (b) a halogenated first flame retardant component which may be a polybrominated diphenyl compound such as decabromodiphenyl oxide or decabromodiphenyl ether; (c) a metal-based second flame retardant component which may be antimony oxide, antimony trioxide, or antimony pentoxide; and (d) a filler component of expandable, intercalated graphite particles such as graphite flake.
Yet another feature of the present invention is to provide a flame retardant, electromagnetic interference (EMI) shielding gasket. Such gasket is constructed as having a resilient core member with a circumferentially outer surface, and an electrically-conductive outer member externally surrounding the core member. The outer member is wrapped over the outer surface of the gasket and has an interior surface at least partially covered by a layer of a flame retardant, pressure sensitive adhesive which bonds the outer member to the outer surface of the core member. The PSA layer is formed of an intumescent PSA composition formulated as an admixture of: (a) a PSA component which may be acrylic-based; (b) a halogenated first flame retardant component which may be a polybrominated diphenyl compound such as decabromodiphenyl oxide or decabromodiphenyl ether; (c) a metal-based second flame retardant component which may be antimony oxide, antimony trioxide, or antimony pentoxide; and (d) a filler component of expandable, intercalated graphite particles such as graphite flake.
Advantages of the present invention includes an intumescent, flame retardant PSA composition which provides UL94 V-0 protection when used as an interlayer to bond a metal foil or electrically-conductive fabric jacket to a polyurethane or other foam core within an EMI shielding gasket construction. Further advantages include a flame retardant, electrically-conductive EMI shielding tape for cable shielding applications. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.